Here is a quick breakdown of why i do extra stuff (besides the fact that I am addicted to working out...lol)
AR--Increase the blood folw to a previously trained muscle therefore speeding the recovery proccess and helping to heal any minor injuries, also makes lactic acid build up almost non existant.
GPP--General Physical Preparedness, I just think of it as constructive conditioning. In other words it is things that will increase your work capacity while at the same time giving you indirect benefits. An increase in work capacity will equate to you being able to train harder and this is a big deal since we know that training longer is not a viable option. Some examples are: sled dragging, car pulling, a lot of gripping exercises, strongman events, high rep olympic lifts, interval sprints, agility drills, plyo's, you get the picture. There is no time for cardio when there are a million skills that need to be developed.
HIIT--Basically use this to develop my anaerobic capacity in my lower body. I tend to focus on change of direction moves as opposed to straight sprints because I feel it has the biggest carryover as far as balance and acceleration are concerned.